1. Field of the Invention
This invention relates to the field materials analysis, and in particular to a transient, rapid spectroscopic method for the analysis of unknown heterogeneous materials by enhanced mixed-wavelength laser plasma spectroscopy.
2. Description of Related Art
Most analytical techniques for the analysis of unknown heterogeneous materials used in industry require samples be taken to the laboratory, and analyzed by time consuming procedures involving instrumentation such as Auger and mass spectrometers, energy-dispersive spectrometry, liquid or gas chromatography, graphite furnace atomic absorption spectroscopy or inductively coupled plasma optical emission spectrometry. Faster in-situ methods such as spark-discharge optical spectrometry are only applicable to electrically conductive materials, while X-ray backscattering probes are limited in sensitivity.
An emerging method, known as laser-induced plasma spectroscopy, promises to provide rapid, in-situ compositional analysis of a variety of materials in hostile environments and at a distance. Basically, this method involves focusing a high-power pulsed laser onto the material, thus vaporizing and ionizing a small volume of the material to produce a plasma having an elemental composition which is representative of the material composition. The optical emission of the plasma is analyzed with an optical spectrometer to obtain its atomic composition. This method has been applied to a variety of materials and industrial environments, as exemplified in the following documents.
U.S. Pat. No. 4,645,342 by Tanimoto et al. describes a probe for spectroscopic analysis of steel including focusing an infrared laser pulse on the steel material and collecting, at an angle of 16 degrees or more, the light emitted by the irradiated surface spot. This probe includes a single laser pulse at 1064 nm not collinear with the collecting optics.
U.S. Pat. No. 4,986,658 by Kim describes a probe for molten metal analysis by laser-induced plasma spectroscopy. The probe contains a high-power laser producing a pulse having a triangular waveshape. In this case, the vaporizing laser beam and collecting optics are collinear, but only a single laser pulse at 1064 nm is used to vaporize the molten metal surface.
U.S. Pat. No. 5,042,947 by Pxc3x6tzschke et al. describes an application of laser-induced plasma spectroscopy for the sorting of solid metal particles, namely shredder scrap from automotive recycling processes. A single laser pulse at 1064 nm is used to produce each laser spark.
U.S. Pat. No. 5,379,103 by Zigler describes a mobile laboratory for in-situ detection of organic and heavy metal pollutants in ground water. Pulsed laser energy is delivered via fiber optic media to create a laser spark on a remotely located analysis sample. The system operates in two modes, one is based on laser-induced plasma spectroscopy and the other on laser-induced fluorescence. Again, only single laser pulses at 1064 nm are used to analyze pollutants in ground water.
U.S. Pat. No. 5,847,825 by Alexander discloses a LIBS system using a femtosecond laser pulse for plasma generation. U.S. Pat. No. 5,757,484 by Miles et al. describes a subsurface soil contaminant identification system using a cone penetrometer based on laser-induced breakdown spectrometry.
In all of the above patents, single laser pulses based on one wavelength are used to vaporize, ionize and excite a portion of the material to be analyzed by laser-induced plasma emission spectroscopy.
The Japanese patent JP62-41999 by Yamamoto et al. discloses a spectral analytical method by laser emission using two step excitation methods. In this case, the second laser beam is perpendicular to the first laser beam and parallel to the target. The two pulses used are at the same wavelength and separated by a specific delay time. The JP62-41999 patent does not use multiple-wavelength laser pulses.
The Japanese patent JP62-85847 by Takaharu describes a method for direct emission spectrochemical analysis of laser multistage excitation. The laser pulse is divided into two pulses P and Q at the same wavelength by a beam splitter. The first pulse P converges on the sample to produce the plasma and then the second pulse Q delayed by an optical delay device converges on the plasma to enhance the light emission. Two pulses at the same wavelength separated with a specific time by an optical delay device generate the plasma on the sample.
In order to enhance sensitivity of laser-induced plasma spectroscopy; several patents, such as JP62-188919 and JP1-321340, generate the plasma by laser double pulse mode. Again, the two pulses are separated by a specific delay time and there is no mention of multiple wavelengths to generate the plasma.
Two temporally close sparks induced by two collinear lasers at 1064 nm are used by Cremers et al. in U.S. Pat. No. 4,925,307 for the spectrochemical analysis of liquids. The sparks occur in the volume of the liquid. The spark produced by the first laser pulse produces a bubble in the liquid which stays in the gaseous state for hundreds of microseconds after the first spark has decayed, so that the second laser pulse, fired typically 18 microseconds after the first pulse, produces a second spark within the gaseous bubble. The emission spectrum of the second spark is detected by a spectrometer oriented at 90 degrees from the laser beam axis. A much increased detectability of the atomic species is obtained by sampling the bubble with the second laser spark. The two laser pulses are at the same wavelength and separated by a time delay.
U.S. Pat. No. 6,008,897 by Sabsabi et al. discloses a method and apparatus for enhanced laser-induced plasma spectroscopy using two pulses of different wavelengths. The first laser pulse, for example in the ultraviolet, vaporizes a small volume at the surface of the material and produces a plasma which is subsequently enhanced by the second laser pulse, for example in the near-infrared. The two pulses are temporally spaced by a predetermined time period; they are not simultaneous as in the present invention.
All work related to laser-induced plasma spectroscopy in the literature uses a single-wavelength laser pulse or double laser pulses separated by a period of time.
According to the present invention there is provided a method of analyzing the composition of heterogeneous materials comprising providing a sample of a material to be analyzed; directing a mixed-wavelength laser pulse at said sample to produce a plasma; and determining the composition of said material from the emission spectrum of said plasma.
The invention provides a method and apparatus to enhance the analytical sensitivity of laser-induced plasma spectroscopy and to provide a reliable analysis of the surface of materials by using a mixed-wavelength laser pulse. In conventional laser-induced plasma spectroscopy, the plasma is produced by a single laser pulse or double pulses separated by a given period of time. A shorter wavelength laser pulse is better absorbed by the sample than a longer wavelength laser pulse, while the longer wavelength pulse is efficiently absorbed by the plasma. In particular, a longer wavelength pulse is absorbed by the portion of the plasma spark which faces the incoming laser beam. In the present invention, a mixed-wavelength laser pulse produces the plasma. The shorter wavelength component of the laser pulse ablates efficiently the sample and rapidly produces an initial plume of ablated matter containing seed electrons. The longer wavelength component of the laser pulse is then well absorbed in the initial plume and further ionizes the ablated matter, rapidly producing a plasma sufficiently dense to prevent the laser from reaching the material surface. Thereafter, the shorter wavelength component of the laser pulse warms up the part of the plasma closest to the sample because the outer portion of the plasma is less absorbing at this wavelength. Meanwhile, the portion of the plasma spark which faces the incoming laser beam absorbs the longer wavelength component of the laser pulse whose wavelength is efficiently absorbed by the plasma. The result is a plasma spark of high, uniform and sustained temperature and emissivity, enabling one to record a plasma emission signal which is orders of magnitude stronger than what would be obtained with a single laser pulse of similar energy. This can be achieved by combining two simultaneous pulses from separate lasers.
Maintaining alignment of the two focal spots can be difficult, and drift in this alignment can significantly affect emission signals and efficiency of sensitivity enhancement. Another aspect of the present invention is the production of mixed-wavelength pulses with a single laser that is provided with one or several frequency conversion devices such as frequency doubling crystals or optical parametric oscillators. There are several advantages of using a single laser. This includes a more compact, reliable, and less expensive system than can be achieved using two lasers. Moreover, no alignment of two laser beams on the sample is required; the wavelength combination is fixed and assured.
The invention also provides an apparatus for use in the analysis of heterogeneous materials comprising at least one laser for generating a mixed-wavelength pulsed laser beam; and a focusing arrangement for focusing said mixed wavelength laser beam onto a sample to produce a plasma.
In one embodiment two the outputs of two lasers operating at different wavelengths are combined. In another embodiment the output of a single laser is split into separate beams. After the frequency of one of the separate beams has been changed, they are again combined into a single mixed wavelength beam.
In a still further embodiment, a frequency converter is included in the output beam of a single laser so as to generate at least one new frequency while retaining the original frequency so as to produce the mixed wavelength beam.